Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. In fact, ozone is the fourth strongest oxidizing chemical known, having an oxidation potential of 2.07 volts. Because of this property, ozone and/or fluid mixtures including ozone are capable of removing a wide variety of contaminants, such as for example cyanides, phenols, and detergents, from surfaces. Also, ozone and/or fluid mixtures including ozone are capable of oxidizing surfaces. In particular, ozonated water is used to "clean", i.e., oxidize, the surface of silicon wafers in-process in the semiconductor industry. Additionally, ozone is also useful for inhibiting, reducing and/or eliminating the accumulation of undesired materials, such as biomass, mold, mildew, algae, fungi, bacterial growth and scale deposits in various aqueous solution systems. When used in this manner, ozonation provides the advantage of producing a lesser quantity of potentially harmful residues than, e.g., chlorination, which leaves undesirable chlorinated residues in aqueous systems.
Because of this wide range of activity, ozone finds application in many diverse processes. Ozone, for example, has been used as a biocide for the treatment of drinking water. Additionally, ozone is used for sterilization in the brewing industry, and for odor control purposes in the sewage treatment industry. Ozonated water also finds wide utility in the semiconductor industry, where for example, ozone is used to clean and surface condition in-process silicon wafers. Additionally, as is described in U.S. Pat. No. 5,378,317, ozonated water is used to remove organic materials, such as photoresist, from the surface of silicon wafers. Moreover, ozonated water is used in the semiconductor industry to form a thin, passivating oxide layer on the surface of silicon wafers.
The use of ozonated water provides several advantages in these applications. First of all, because ozonated water is generated at the point of use, it is relatively free of contaminants, i.e., particles and metals, that are typically present in chemicals that are stored in barrels or drums. Ozonated water is also less expensive than other oxidizing chemicals and furthermore, since ozonated water naturally decomposes, the use of ozonated water presents few disposal issues. However, the effectiveness of ozone in each of these applications is adversely affected by its low solubility and short-half life (approximately 10 minutes) in aqueous solutions. That is, not only is it difficult to dissolve ozone in an aqueous solution, but also, once dissolved, it is difficult to maintain the ozone in solution.
Several methods of increasing the quantity of dissolved ozone in aqueous solutions are known. For example, bubbling ozone directly into water at ambient pressure has been used as a method to dissolve ozone in aqueous solutions. Additionally, published European patent application No. EP 0 430 904 A1 discloses a process for producing ozonated water comprising the step of contacting, within a vessel of defined volume, an ozone-containing gas with fine droplets of water. Methods utilizing cooling to increase the quantity of dissolved ozone in aqueous solutions have also been proposed. For example, U.S. Pat. No. 5,186,841 discloses a method of ozonating water comprising injecting ozone through an aqueous stream across a pressure drop of at least 35 psi. The ozonated stream is then combined with a second stream that is preferably a portion of an aqueous solution which is recirculating in a cooling water system. The resultant stream is forced to flow at a velocity of 7 feet per second for a distance sufficient to allow 70% of the ozone to be absorbed. Additionally, U.S. Pat. No. 4,172,786 discloses a process for increasing the quantity of dissolved ozone in an aqueous solution by injecting an ozone containing gas into a side stream conduit which circulates a portion of cooling water. The ozone-injected water is then mixed with the cooling water in a tower basin, thereby ozonating the water. Finally, U.S. Pat. No. 5,464,480 discloses a process for removing organic materials from semiconductor wafers using ozonated water. Specifically, this patent teaches that high ozone concentration water, suitable for use in the disclosed process may be obtained by mixing ozone and water at a temperature of from about 1.degree. C. to 15.degree. C.
An improved method for increasing and maintaining the quantity of ozone dissolved in a liquid is described in commonly assigned U.S. Pat. No. 5,971,368, the entire disclosure of which is incorporated by reference in its entirety for all purposes. The '277 application discloses a pressurized vessel into which a stream of gas, such as ozone, is introduced to contact and dissolve in an amount of liquid, such as ultrapure deionized water, and an outlet that dispenses a stream of the ozonated liquid under sufficiently gentle conditions such that an increased quantity of ozone dissolved in the liquid stream is dispensed.
Spray processing tools and methods for dispensing a stream of liquid used to clean a substrate surface are well known and in use in the a variety of industries, including the semiconductor industry. For example, the MERCURY.RTM. centrifugal spray processor, commercially available from FSI International, Chaska, Minn., includes a horizontal turntable that supports one or more semiconductor wafers and a vertical spray post that dispenses one or more processing chemicals. The processor includes vertical supports on the turntable for supporting semiconductor wafer cassettes. The spray post is fluidly coupled to one or more reservoirs containing the chemical(s) to be dispensed, and includes one or more nozzles positioned on the spray post out of which each chemical is dispensed. The chemical exits the spray post nozzle in a direction that is perpendicular to the vertical spray post and parallel to the horizontal turntable. The processing chemical impacts the one or more semiconductor wafers supported by the turntable to carry out the desired treatment. The turntable can be rotated independent of the spray post to distribute the stream of processing chemical over the semiconductor wafer. Such a processing tool is advantageously used, for example, to strip photoresist from the semiconductor wafer. The use of ozonated water for such a photoresist stripping application is widely used due to the oxidative qualities of ozone described above.
The strip rate in such an application is dependent not only on the ozone content in the liquid stream, but it is also strongly dependent upon the flow rate of the ozonated water over the substrate processed by the stream of ozonated water, i.e. the semiconductor wafer. This suggests that increasing the flow rate of the ozonated water over the substrate to be processed should improve the strip rate of the ozonated water. One method for increasing the flow of ozonated water over the substrate surface is to increase the rate at which the stream of ozonated water is dispensed from the spray post. As described above, however, it can be difficult to maintain the concentration of ozone dissolved in a liquid, and the conditions under which the ozonated water is dispensed can directly affect the concentration of the ozone in the liquid stream. Accordingly, increasing the flow of ozonated water from an ozonated water generation system (such as is described above) typically reduces the concentration of ozone in the ozonated water stream. This, in turn, can negatively impact the strip rate rather than increasing the strip rate as is desired.
Thus, there is a continuing need for a more efficient processing tool and method of dispensing a stream of processing fluid, such as ozonated water, at a point of use. In particular, a tool and method that increases the efficiency of the processing fluid, such as for example a tool and method that increases the strip rate of the ozonated water, would be especially advantageous.